Modems

This document describes modems, and the technologies they utilize. A modem converts digital signals from a computer to analog signals so they can be sent over a telephone line. The receiving computer will normally have a modem which will convert the analog signals back to digital signals. A modem may be an internal modem which is a card inside your computer or it may be external with a connection to a serial RS-232 line on your computer. The telephone jack that will plug into the modem is called an RJ-11 jack. Speed of the modem is measured in bits per second (bps). There are a set of V-series standards developed by the International Telecommunications Union which indicate the speed of the modem.

• V.22bis - 2400bps
• V.32 - 9600bps
• V.32bis -14,400bps
• v.32terbo - 19,200bps
• V.FastClass (V.FC) - 28,800bps
• V.34 - 28,800bps
• V.42 - 57,600bps

These are data compression standards which enable the transmission to operate at a higher speed. Due to data compression used on modems in recent years baud rates and bps when referring to modem speed are no longer the same. Now, more than one bit can be sent with each sound wave oscillation (baud). In the past, only one bit could be sent per sound wave oscillation.

Modem Types

• Asynchronous - The common modem used today. Each byte is placed between a stop and a start bit. Each modem must operate with the same start and stop bit sequence, operate at the same baud rate and have the same parity settings for the data checking in order to communicate correctly. Define parity checking.
• Synchronous - They depend on timing to communicate. They "transmit data in frames with occasional synchronization (or sync) bits inserted to ensure the accuracy of timing." They are normally used on dedicated lines that are leased.
• Digital Modems - These are used with ISDN services and are not actually modems, although they are called modems. They can provide connection speeds of 128Kbps.